Refactor EditorBottomPanel using theme system

Author: lodetrick

editor/editor_node.cpp

@@ -575,7 +575,6 @@ void EditorNode::_update_theme(bool p_skip_creation) {
 
 		editor_main_screen->add_theme_style_override(SceneStringName(panel), theme->get_stylebox(SNAME("Content"), EditorStringName(EditorStyles)));
 		bottom_panel->add_theme_style_override(SceneStringName(panel), theme->get_stylebox(SNAME("BottomPanel"), EditorStringName(EditorStyles)));
-		bottom_panel->get_back_panel()->add_theme_style_override(SceneStringName(panel), theme->get_stylebox(SNAME("BottomPanel"), EditorStringName(EditorStyles)));
 		distraction_free->set_button_icon(theme->get_icon(SNAME("DistractionFree"), EditorStringName(EditorIcons)));
 		distraction_free->add_theme_style_override(SceneStringName(pressed), theme->get_stylebox(CoreStringName(normal), "FlatMenuButton"));
 
@@ -8024,6 +8023,7 @@ EditorNode::EditorNode() {
 	// Bottom panels.
 
 	bottom_panel = memnew(EditorBottomPanel);
+	bottom_panel->set_theme_type_variation("BottomPanel");
 	center_split->add_child(bottom_panel);
 	center_split->set_dragger_visibility(SplitContainer::DRAGGER_HIDDEN);